Conventionally, production of a steel sheet pile having joints at both ends of a hat-shaped shape or a U-shaped shape is performed by a caliber rolling method. Known as a general process of the caliber rolling method is first heating a raw material to a redetermined temperature in a heating furnace and sequentially rolling the raw material by a rough rolling mill, an intermediate rolling mill, and a finish rolling mill including calibers.
According to the above-described general caliber rolling method, a domestically produced steel sheet pile product can be produced from a raw material in a rectangular cross-section in status quo. Concretely, for example, a hat-shaped steel sheet pile product called a 10H product having a cross-section second moment per 1 m of a wall width of 1.0 (104 cm4/m) and a hat-shaped steel sheet pile product called a 25H product having a cross-section second moment per 1 m of a wall width of 2.5 (104 cm4/m) are produced by the conventionally known general caliber rolling method.
In the case of producing the steel sheet pile from the raw material in a rectangular cross-section, it is known that various shape defects occur in a material to be rolled in its rolling step, and a solution therefore is devised. For example, Patent Document 1 discloses a technique of applying heavy reduction to a bite end part in order to suppress the occurrence of a bite shape at an end part flange of the material to be rolled in rolling and shaping. Besides, Patent Document 2 discloses a technique of suppressing the occurrence of a crop by forming a tip end part of the material to be rolled before rough rolling in production of shape steel. Besides, Patent Document 3 discloses a technique of imparting a preforming part shape to the end part of the material to be rolled in order to decrease the crop.